The field of the disclosure relates generally to 3D-imaging used to obtain ranging information of objects in the field, and more specifically to methods and systems that facilitate a reduction in photon crosstalk between individual pixels of an avalanche photodiode detector array.
Objects may be inspected, for example, to determine a size and/or shape of all, or a portion, of the object and/or to detect defects in the object. For example, known gas turbine engine components, such as turbine or compressor blades, are routinely inspected to detect fatigue cracks that may be caused by vibratory, mechanical, and/or thermal stresses induced to the engine. Moreover, and for example, some gas turbine engine blades are inspected for deformations such as platform orientation, contour cross-section, bow and twist along a stacking axis, thickness, and/or chord length at given cross-sections. Over time, continued operation of a component with one or more defects may reduce performance of the component, and/or lead to component failures, for example, as cracks propagate through the component. Accordingly, detecting defects within components, as early as possible, may facilitate increasing the performance of the system in which the component is incorporated and/or reducing component failures.
To facilitate 3-D imaging, a laser beam of a laser detection and ranging (LADAR) system is projected onto the surface of the target. An array of photodiode detectors receive light reflected from the surface of the target and a series of amplifiers and a processor record the positions of light detectors within the array that detected the reflected light and the corresponding travel time of the pulse of light to each of the detectors. From this information, the processor develops a 3D image of the target.
LADAR operating in the short wavelength infrared (SWIR) region (1.0-1.6 μm) is a promising tool for 3D-imaging and enabling precision target identification under low-light-level conditions. An avalanche photodiode (APD) focal plane array (FPA) is an example of a LADAR detector capable of single-photon level detection and imaging. However, because the APD pixels are biased very close to or even beyond a nominal breakdown voltage, they are sensitive to incident photons. A single charge carrier (i.e., a single photon) injected into a depletion layer of the APD pixel triggers an avalanche breakdown, which produces electrons and a resulting avalanche current. The leading edge of the avalanche pulse corresponds to the arrival time of the detected photon. When the APD FPA is used for imaging, the arrival time of the detected photon is used to create an image. However, with many carriers in the depleted region, any recombination in this process may generate secondary photon emission, which may be captured by other biased pixels in the array. It would be desirable to optically isolate each pixel of the array to facilitate prevention of ruined imaging due to secondary photon emissions.